Stents in general are endovascular prostheses and/or implants which are used for treating stenoses, for example. Stents are additionally known for the treatment of aneurysms. Stents fundamentally have a support structure which is capable of supporting the wall of a vessel to widen the vessel and/or bypass an aneurysm.
For this purpose, stents are inserted in a compressed state into the vessel and then expanded at the location to be treated and pressed against the vascular wall. This expansion may be performed with the aid of a balloon catheter, for example. Alternatively, self-expanding stents are also known in the art. These stents are constructed from a superelastic metal, such as Nitinol, for example.
Stents are currently classified in two main types, the nondegradable (permanent) stents and the (bio)degradable stents. Nondegradable stents are designed such that they remain in the vessel for an unspecified period of time. (Bio)degradable stents (hereinafter referred to as “degradable” stents), in contrast, are degraded over a predetermined period of time in a vessel. Degradable stents are preferably first degraded when the traumatized tissue of the vessel is healed, a support function is no longer necessary, and the stent no longer has to remain in the vascular lumen.
However, it has been shown that side effects, such as restenosis and thromboses, may occur due to the introduction of stents into vascular systems.
For this reason, stents have been developed which contain one or more agents which are discharged to the organism, as well as to the support structure, upon or after implantation in the vascular system.
Known stents coated with agent have a coating, the support structure of the stent typically being coated with an agent-containing polymer on both the luminal and also the abluminal (mural) surface.
The release of the agents from the polymer matrix typically results via diffusion processes and/or erosion processes of the polymer matrix.
In stents whose luminal and abluminal surfaces have been coated with growth-inhibiting antiproliferative agents, it has been established that the endothelialization of the stent (overgrowth of the stent with vascular cells) is slowed or prevented and the risk of a thrombosis is thus increased.
In a stent having magnesium alloy, the pH value of the magnesium stent implanted in a vessel rises in the immediate surroundings of the support structure because of the Mg(OH)2 formation as a result of the degradation. Such a pH value shift into the basic range may be particularly harmful for pH-unstable agents, in particular, rapamycin and paclitaxel.
If polymer material (e.g., polyester) degradable by hydrolysis is used as the matrix for the agent coating, the pH value of a stent implanted in a vessel sinks as a result of the hydrolysis of the degradable polymer and formation of the corresponding acids (lactic acid, glycolic acid, and the like). In particular, the decomposition speed of a degradable metal stent main body may be negatively influenced by the pH value shift into the acid range.